An ultrasonic transducer is used to diagnose a tumor in a human body or the like by transmitting and receiving ultrasonic waves.
Although an ultrasonic transducer utilizing vibrations of a piezoelectric member has been used up to now, a capacitive micromachined ultrasonic transducer (CMUT) whose vibration portion is formed on a silicon substrate has been under extensive development for its practical use according to the advance of the MEMS technology in recent years.
U.S. Pat. No. 6,320,239B1 specification (Patent Document 1) discloses a single CMUT and CMUTs arranged in an array.
U.S. Pat. No. 6,571,445B2 specification (Patent Document 2) and U.S. Pat. No. 6,562,650B2 specification (Patent Document 3) disclose a technology for forming a CMUT on an upper layer of a signal processing circuit formed on a silicon substrate.
“2003 IEEE ULTRASONIC SYMPOSIUM” pp. 577 to 580 (Non-Patent Document 1) discloses a technology in which a lower electrode is formed to be larger than a cavity layer in a CMUT.